The overall aim of Core B, the Optics and PDT Dosimetry Core, is to provide support for the light delivery for PDT both in the clinic and in the laboratory. This include the setup for the light delivery devices (lasers, fibers, beam splinters) for both clinical and laboratory photodynamic therapy. PDT in both the laboratory and clinic requires proper dosimetry. The Core provides equipment and personnel to support the research of the projects and works to maximize the yield of useful information. To perform this function it is necessity to understand the dosimetry of PDT. This leads an effort in modeling. The limited understanding of dosimetry as dependent on the light and a drug has been expanded through study of the QSAR data on the pyropheophorbide hexyl ether series to include a sensitive target for PDT. We intend to continue to study the dosimetry of PDT in an attempt to support and provide a testable model framework in which to understand the results of the other projects in the overall study of structure activity relationships. Core personnel will perform laser maintenance, calibration and dosimetric measurements required by the clinical and laboratory research activities of the projects. The specific aims of Core B are: i. Technical support for clinical and scientific lasers; ii. Measurement of photobleaching during therapy; iii. Measurement of the total attenuation of tumor and normal tissue in patients undergoing PDT for the treatment of skin tumors; iv. Study of the pharmacokinetics of pyropheophorbide hexyl ether and subsequent series and ALA; v. PS distributions in frozen sections of tumors and normal tissues; vi. Refining the analytic framework of PDT targeting and dosimetry.